Cathode ray tube blanking circuit for recurrent and triggered sweeps



Oct. 25, 1966 N. E. PETERSON ETAL 3,281,623

CATHODE RAY TUBE BLANKING CIRCUIT FOR RECURRENT AND TRIGGERED SWEEPS Filed Aug. 29, 1963 United States Patent O 3,281,623 CATHODE RAY TUBE BLANKING CIRCUIT FOR RECURRENT AND TRIGGERED SWEEPS Norman E. Peterson and Ernest E. Courchene, Jr., Norwalk, Coun., assignors to Digitech, Inc., South Norwalk, Conn., a corporation of Connecticut Filed Aug. 29, 1963, Ser. No. 305,277 1 Claim. (Cl. 315-22) The present i-nvention relates to a cathode ray tube beam blanking circuit and more particularly to such a circuit for both recurrent and triggered sweeps.

In conventional applications of cathode ray tubes in Oscilloscopes and other indicators, linear indications are obtained by suitable linear deflections of the cathode ray beam. 4I n such applications, the beam is made visible during i-ts linear sweep in one direction and is cut oit during its return sweep. This cut off or blanking of the beam is done .to make t-he retrace path of the beam invisible and also to eliminate a stationary spot which would result at .the standby or initial undeflect-ed position of the beam.

This blanking of the beam is now customarily done by applying a suitably generated blanking voltage to control electrodes Within the tubes such as the control grid. Since high accelerating vol-tages are used in cathode ray tubes the control grid will norm-ally be as much as 1,000 volts or more away from ground potential. For this reason, it is customary to couple the blanking voltages to the control grid or to the cathode through a capacitor coupling. This arrangement is satisfactory for recurring sweep signals, however, such couplings have proven less satisfactory where triggered sweeps are used as the R-C time connstant of such couplings Ibecome prohibitively long for the indefinite time period required with triggered sweeps.

The present invention provides a blanking circuit for this use permitting a capacitor coupling which does not require long R-C time const-ants and which is, therefore, ideally suited for tri-ggered sweep signals where the cathode ray tube beam may be blanked for indefinite periods. The circuit provided obtains this result using normal bleeder currents so that additional power sources are not required and the circuit also operates with no side effects on tube operation as it operates with a constant bleeder current regardless of whe-ther the beam is on or oit.

Accordingly, an object of the present invention is to provide an improved blanking circuit for a cathode ray tube.

Another object of the present invention is to provide an improved cathode ray blan-king circuit adapted for both recurrent and non-recurrent or triggered sweep signals.

Another object of the present invention is to provide an improved cathode ray tube beam blanking circuit particularly su-i-ted for triggered sweep signals.

Another object of the present invention is t-o provide a cathode ray tube beam bl-anking circuit having memory characteristics so that Iit is capable of stable oper-ation over indefinite periods with the beam remaining in the desired on or oif post-iion.

Another obje-ct of the present invention is to provide an improved cathode ray Itube beam blanking circuit operating from the regu-lar tube power system.

Other and further objects of the invention will be obvious upon ,an understanding of the illustrative embodiment about to be described, o-r will be indicated in the appended claim, land various advantages n-ot referred to herein wil-l occur to one skilled in the art upon employment of the invention in practice.

A preferred embodiment of the invention has been chosen for purposes of illustration and description and is shown in the accompanying drawings, forming a part of the specification, wherein:

IFIG. 1 is a schematic diagram of a preferred embodiment of the blanking circuit.

FIG. 2 is a diagrammatic illustration of typical Wave forms for cathode ray tube sweep and blanking signals.

FIG. 1 illustrates a conventional cathode ray tube 1 for use in a typical oscilloscope or similar circuit. The tube 1 illustrated has electrostatic deflection plates 2, however, it is cle-ar that the beam blanking circuit shown in Ikthis figure `and described herein may be use-d for magnetic deflection as well.

The potentials for the several tube electrodes are obtained as illustrated by taps on a bleeder resistance circuit 3 connected between the high voltage negative output fof a high voltage power supply 4 and ground. The focusing electrode 5, the cathode 6, and the anode 7 are connected conventionally .to taps on the bleeder circuit 3 as indicate-d. The blanking signal to turn the beam on and ott for the reasons described above is coupled to the control -grid 8 from the bleeder 3 under the control of the novel parallel branch portion 9 of the bleeder 3 whose arrangement and operation will now be more fully described.

The -catho-de ray beam may be cut on or off by con- `trolling the negative bias voltage on lthe control g-rid relative to the cathode. With a low negative bias applied (i.e. about -10 volts for a typical tube), a visible beam is produced and when the negative bias is increased (i.e. to about 40 volts in la typ-ical circuit), the beam is cut off.

One of the two parallel branches for this grid bias portion 9 of the bleeder circuit 3 comprises a resistor 10 and the collector to emitter resis-t-ance of one transistor 11. The other parallel branch comprises a second resistance 12 ofthe same value as `the first resistor 10 and the collec- '.tor to emitter resistance of a second transistor 13 similar to the rst transistor 11. These two branches includ-ing the transistors are connected into a multivibrator circuit of the bistable type. This is done as illustrate-d by connecting the collector 14 of the rst transistor 11 to the base 15 of the second transistor 13 through the parallel R-C coupling illust-rated at 16 a-nd 17. The collector 18 of t-he second transistor 13 is similarly connected to the base 19 of the -rst transistor by -an identical R-C coupling 20, 21. It is seen that this provides a bist-able multivibrator when a suitable bias is provided for the base elements 15 and 19 through resistors 22 an-d 23.

A blanking bias voltage on the control grid 8 with respect to the cathode 6 is obtained such as the squarewlave illustrated at D in FIG. 2 when timing signals are coupled between the base and the emitters of the .two transistors 11 and 13 by isolating capaci-tors 24 and 25, rectiiiers 26 and 27 and resistors 28 and 29.

A conventional squarewave generator circuit 30 is triggered by a trigger signal A and has i-ts output signal B coup-led to a sweep signal generator such as an integrator 31 whose output C is coupled to the cathode ray tube 1 deflection plates 2. The squarewave signal B which is thus synchronized with the sweep signal output C is also coupled to the multivibrator inputs. At the beginning of each sweep at time a, the positive signal transition of wave B ris coupled to transistor 11 through capacitor 24 and :rectifier 26 turning .this transistor ott and thus by the multivibrator action turning transistor 13 on.- When transistor 11 is turned off its relatively high resistance in eect opens this side of the parallel branches 9 While the extreme-ly low resistance between the collector and t-he emitter of transistor 13 causes the bias voltage on the control grid 8 -to be determined by ythe bleeder 3 including the resistor 12. Resistor 12 is chosen with rel-ation to resistors 33 through 36 to give the required negative bias on the control grid 8 relative to the cathode 6 to turn the beam on. This bias in a typical cathode ray tube may be about 3 volts. The concurrent negative transition on transistor 13 is blocked by rectifier 27.

At the completion of the rst sweep `of the cathode `ray beam and at the time b on the time scale of FIG. 2, the beam is blanked out -by the coupling of the positive signal transition of wave B to transistor 13 through capacitor 25 and rectifier 27 so that this transistor is turned olf while the transistor 12 by multivibrator action is again turned on. With transistor 13 turned olf, its extremely high resistance in effect opens this side of the parallel resista-nce branches 9. The turning on of the transistor 12 at the same time in eiect couples resistor 10 to resistor 33. It is thus clear that the control grid 8 is now coupled in effect to the high voltage negative output causing the control grid to be suciently below the cathode potential to blank out the beam, for example, providing a bias voltage of about -40 volts in a typical cathode ray tube.

It is thus clear that the above described bistable circuit will remain in either one or the other of the above described conditions depending on the sweep trigger signal A. A recurrent blanking is obtained by the use of a recurring sweep trigger signal or a non-recurrent or triggered type of operation is obtained using an appropriately timed trigger signal. The beam will remain on its yon or off condition for indefinite periods depen-ding on its most lrecently fed control signal.

The preferred embodiment has been illustrated and described for a multivibrator circuit using two transistors for the switching means. The use of transistors is particularly desirable ydue to their low power requirements. It is clear, however, that the equivalent vacuum tube multivibrator circuit may be used in place lof the above described transistor to provide the switching action in the bleeder circuit.

It will be seen that the above described blanking circuit provides for an improved cathode ray beam control adapted for both recurrent and non-recurrent or triggered signals. This improved result is `obtained in a relatively simple circuit which operates on the regular high voltage bleeder power supply for the cathode ray tube so that no ladditional power circuitry is required. The circuit is also symmetrical in all respects giving constant bleeder current for on and off beam signals providing stability for the other electrodes of the cathode ray tube coupled to the bleeder resistance. In addition, a blanking circuit is provided which will hold the beam in its selected on or off state, as the case may be, for indenite periods.

As various changes may 4be made in the form, construction and arrangement of the parts herein without departing from the spirit and scope of the invention and without sacricing any of its advantages, it is to be understood that all matter herein is to be interpreted as illustrative and not in a limiting sense.

Having thus `described our invention, we claim:

In a control circuit for a cathode ray tube having an electron rbeam whose intensity is controlled by electrode bias voltage obtained from a bleeder resistance the improvement which comprises said bleeder resistance being coupled between a negative high voltage source and ground, a -pair of transistors each having a collector and emitter and base, the end portion of said bleeder resistance at the high voltage end comprising parallel branches each including a resistor having one end coupled to the high voltage source and having its opposite end connected through the collector to emitter resistance of lone of said transistors to ground through a common bleeder `resistance, the collector of each transistor being coupled to the base of the Iother transistor through a parallel resistor capacitor network forming a bistable multivibrator, said cathode ray tube having a control grid coupled to the collector of one of said transistors, a square wave voltage source having one output coupled to one transistor base and its other output coupled to the other transistor base, and the cathode `of said cathode ray tube being coupled to said common bleeder resistor.

References Cited by the Examiner UNITED STATES PATENTS 2,419,118 4/ 1947 Christaldi 315-22 4/1959 Solow 315-22- 

